Where is Grease Pencil stroke snap mode in UI?
Thank you for posting your question very clearly.
This is the reason why SNAP FIRST and SNAP LAST and LOOSE SNAP exist.
This was recently implemented.
When you START a drawing using the circle snap your drawing will conform to the first and last stroke point when you let go of the draw. If you draw and CLOSE the shape the snap will conform to first point to snap. And obviously if you don´t do any of these, you´ll have LOOSE snap. 

I´m glad that you´re reading into this inside the API.
Thank you for your vote for this answer if you find what you were looking for

BFCT_Schiller